Time stamp correction apparatus and time stamp correction system

ABSTRACT

There is provided a time stamp correction apparatus for being used in an apparatus which receives a packet including real time data and a time stamp based on network time from a network and reproduces the real time data on the basis of the time stamp, the time stamp correction apparatus including: receiving a first packet including the real time data and a first time stamp; successively generating the network time according to synchronous information input from the network; calculating a difference between time by the first time stamp and the network time; successively generating local time; generating a second time stamp based on the difference and the local time; generating a second packet by adding the second time stamp to the real time data; storing the generated second packet into a buffer unit; and conducting output control on the second packet on the basis of the local time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35USC § 119 toJapanese Patent Application No. 2004-96194 filed on Mar. 29, 2004, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a time stamp correction apparatus and atime stamp correction system.

2. Related Background Art

In some cases, devices connected to a network share time information(network time). If there are a plurality of networks, unique networktime is administered to every network.

A device transmitting real time data (transmitting device) generatesinformation of time (time stamp) for the real time data to be reproducedin a receiving device, on the basis of network time in a network towhich the transmitting device belongs itself. This time stamp iscalculated by adding an offset value to the network time. Thetransmitting device transmits the generated time stamp to the receivingdevice together with the real time data as a real time packet.

After receiving the real time packet, the receiving device processesreal time data synchronized with the network time, on the basis of thetime stamp contained in the real time packet. The receiving devicetemporarily retains the received real time packet in an internal storagedevice (FIFO (First In First Out)) until data processing is started.

The network (first network) to which the above-described transmittingdevice and receiving device belong is connected to (integrated with) adifferent network (second network) in some cases. Network time in thenetwork after integration is unified into the network time of either thefirst network or the second network. If the network time afterintegration is made equal to the network time of the second network,discontinuity occurs in the network time of the first network includingthe above-described transmitting device and receiving device.

If in this case a real time packet is already retained in the FIFO inthe receiving device at the time of the network integration, a timestamp in this real time packet is based on the old network time beforethe integration. This results in a problem that the receiving devicecannot continue data processing while maintaining real time quality.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided atime stamp correction apparatus which receives a packet including realtime data and a time stamp, which is based on network time and to beadded to the real time data from a network, and reproduces the real timedata in the received packet on the basis of the time stamp, the timestamp correction apparatus comprising: a packet receiving unit whichreceives a first packet including the real time data and a first timestamp based on the network time from the network; a network time counterwhich successively generates the network time in accordance with networktime synchronous information input from the network; a differencecalculating unit supplied with the first time stamp included in thereceived first packet to calculate a difference between time indicatedby the first time stamp and the network time; a local time counter whichsuccessively generates local time; a time stamp generating unit whichgenerates a second time stamp on the basis of the difference and thelocal time; a packet generating unit which generates a second packet byadding the second time stamp to the real time data; a buffer unit whichstores the generated second packet; and an output control unit whichconducts output control on the second packet in the buffer unit on thebasis of the local time.

According to an aspect of the present invention, there is provided atime stamp correction system for being used in an apparatus whichreceives a packet including real time data and a time stamp, which isbased on network time and to be added to the real time data from anetwork, and reproduces the real time data in the received packet on thebasis of the time stamp, the time stamp correction system comprising: atime stamp correction apparatus comprising a packet receiving unit whichreceives a first packet including the real time data and a first timestamp based on the network time from the network, a network time counterwhich successively generates the network time in accordance with networktime synchronous information input from the network, a local timecounter which successively generates local time, and a differencerequesting unit which requests calculation of a difference between thefirst time stamp in the first packet and the network time; and a CPUconnected to an identical bus with the time correction apparatus toexecute the difference calculation requested by the time stampcorrection apparatus and output an execution result, wherein the timestamp correction apparatus further comprises: a time stamp generatingunit which receives the execution result from the CPU and generates asecond time stamp on the basis of the received execution result and thelocal time; a packet generating unit which generates a second packet byadding the second time stamp to the real time data; a buffer unit whichstores the generated second packet; and an output control unit whichconducts output control on the second packet in the buffer unit on thebasis of the local time.

According to an aspect of the present invention, there is provided atime stamp correction system for being used in an apparatus whichreceives a packet including real time data and a time stamp, which isbased on network time and to be added to the real time data from anetwork, and reproduces the real time data in the received packet on thebasis of the time stamp, the time stamp correction system comprising: atime stamp correction apparatus comprising a packet receiving unit whichreceives a first packet including the real time data and a first timestamp based on the network time from the network, a network time counterwhich successively generates the network time in accordance with networktime synchronous information input from the network, a local timecounter which successively generates local time, a differencecalculating unit supplied with the first time stamp included in thefirst packet to calculate a difference between the first time stamp andthe network time, and a time stamp generation requesting unit whichrequests generation of a second time stamp based on the difference andthe local time; and a CPU connected to an identical bus with the timecorrection apparatus to generate the second time stamp with accordancewith the request from the time stamp correction apparatus, and outputthe generated time stamp, wherein the time stamp correction apparatusfurther comprises: a packet generating unit which receives the secondtime stamp and generates a second packet by adding the real time data tothe second time stamp; a buffer unit which stores the generated secondpacket; and an output control unit which conducts output control on thesecond packet in the buffer unit on the basis of the local time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a time stampcorrection apparatus according to an embodiment of the presentinvention;

FIGS. 2A and 2B are diagrams schematically showing structure examples ofa real time packet;

FIGS. 3A and 3B are diagrams schematically showing structure examples ofa packet including an original time stamp;

FIGS. 4A and 4B are diagrams showing a problem caused when networksrespectively having different cycle times (network times) are connectedto each other;

FIG. 5A is a block diagram showing a configuration of a receiving devicethat has been implemented before the present inventor achieves thepresent invention, and FIG. 5B is a diagram showing how discontinuityoccurs in network time;

FIG. 6 is a diagram showing an effect obtained according to the presentembodiment;

FIG. 7 is a block diagram showing a configuration of a time stampcorrection system according to an embodiment of the present invention;

FIG. 8 is a block diagram showing a configuration of a time stampcorrection system according to another embodiment of the presentinvention; and

FIG. 9 is a block diagram showing a configuration of a time stampcorrection system according to further another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram showing a configuration of a receiving device(time stamp correction apparatus) 10 according to an embodiment of thepresent invention. This receiving device 10 is used in, for example, anapparatus having an IEEE 1394 serial interface. The receiving device 10executes the so-called isochronous data transfer with another deviceconnected to an IEEE 1394 network. Hereafter, the receiving device 10will be described in detail.

As shown in FIG. 1, a cycle time counter 11 successively generates andoutputs a cycle time CT (network time). The cycle time counter 11generates the cycle time (network time) on the basis of a cycle startpacket CSP (network time synchronous information) received from thenetwork.

This cycle start packet CSP is generated by a cycle master (see FIG. 4A)connected to the same network. The cycle master notifies respectivedevices of the generated cycle start packet CSP periodically (forexample, at intervals of 125 μs). As the cycle master, for example, aroute node of a device connected to the network is selected.

A receiving filter 12 receives a real time packet ISOP sent from thetransmitting device (not illustrated), via the network.

FIGS. 2A and 2B are diagrams schematically showing structure examples ofthe real time packet ISOP.

Either of the structures includes two headers IPH and CIPH, a time stampTS, and real time data DT. The real time data DT is, for example,multimedia data such as image data or sound data. A packet structureshown in FIG. 2A further includes a reserved region.

Referring back to FIG. 1, the receiving filter 12 removes the headersIPH and CIPH from the real time packet ISOP, separates the packetresulting from the removal into the real time data DT and the time stampTS, and outputs them (in the case of the packet structure shown in FIG.2B). (In the case of the packet structure shown in FIG. 2A, thereceiving filter 12 separates the packet resulting from the removal intothe real time data DT with the reserved region and the time stamp TS,and outputs them. The following description is premised on the packetstructure shown in FIG. 2B.)

A difference unit 13 successively receives the cycle time CT from thecycle time counter 11. Furthermore, the difference unit 13 receives thetime stamp from the receiving filter 12. Upon receiving the time stampTS, the difference unit 13 calculates a difference (magnitude ofdifference) between the time stamp TS and the cycle time CT at thattime, such as, for example, “time stamp TS—cycle time CT.” In otherwords, the difference unit 13 calculates a time interval between thecurrent network time and the time when to output the real time data DTfrom a receiving FIFO 17 described later. The difference unit 13 outputsthe calculated difference to an adder 14 as an offset OS.

The adder 14 successively receives an original time (internal time) OTfrom an original time counter 15. The original time (internal time) OTcorresponds to, for example, local time. The original time counter 15counts individual original time (internal time) OT independently of thenetwork time.

In other words, the cycle time counter 11 counts the network time basedon the cycle start packet CSP. For example, if a network to which thecycle time counter 11 belongs is connected to a different network andconsequently its own network time is united to the network time of thedifferent network, the cycle time counter 11 starts to count a newnetwork time on the way. In other words, the cycle time counter 11starts the network time that is discontinuous with the original networktime. On the other hand, the original time counter 15 counts theoriginal time (internal time) unique to only the present apparatus.Unless reset (for example, unless the counter reaches its maximum),therefore, the original time (internal time) does not becomediscontinuous.

Upon receiving the offset OS from the difference unit 13, the adder 14adds the offset OS and the original time OT at this time and therebygenerates a new time stamp (original time stamp OTS). The original timestamp OTS indicates a time interval between the current internal timeand the time when the real time data DT is output from the receivingFIFO 17 described later. If the original time stamp 15 counts down theoriginal time OT as represented by, for example, 10, 9, 8 . . . , theadder 14 subtracts the offset OS from the original time OT to generatethe original time stamp OTS.

A data generating unit 16 receives real time data DT from the receivingfilter 12. On the other hand, the data generating unit 16 receives theoriginal time stamp OTS from the adder 14. The data generating unit 16adds the original time stamp OTS to the real time data DT, therebygenerates a new packet OTS&DT, and inputs the packet OTS&DT to thereceiving FIFO 17. The receiving FIFO 17 successively stores the inputpacket OTS&DT. Generated packets OTS&DT are shown in FIGS. 3A and 3B soas to be respectively associated with the structures shown in FIGS. 2Aand 2B.

An output control unit 18 successively receives the original time OTfrom the original time counter 15, and compares the received originaltime OT with the original time stamp OTS in a head packet OTS&DT in thereceiving FIFO 17. If the original time stamp OTS reaches the originaltime OT, the output control unit 18 removes the original time stamp OTSfrom the packet OTS&DT. (In the case of the packet structure shown inFIG. 2A, reserved region data is further removed.) Thereafter, theoutput control unit 18 outputs the real time data DT to a subsequentstage (for example, a data reproducing unit which is not illustrated).

Effects obtained in the present embodiment will now be described.

FIGS. 4A and 4B are diagrams showing a problem caused when networks W1and W2 respectively having different cycle times (network times) areconnected to each other. Each of nodes 0 to 4, 10 and 11 in the networksW1 and W2 includes a receiving device 109 (see FIG. 5A) described laterthat has been implemented before the present inventor achieves thepresent invention.

As shown in FIG. 4A, the network W1 has a cycle time N, and the networkW2 has a cycle time M. Nodes 0 and 10 in the networks W1 and W2 arecycle masters. Each of the cycle masters 0 and 10 periodically notifiesnodes connected to its own network of the cycle time.

It is now supposed that the networks W1 and W2 are connected to eachother at time T1 to form a network W3. If the network configuration ischanged, selection of the cycle master is newly conducted. It is nowsupposed that the node 0 is selected as a cycle master of the networkW3. Therefore, the node 0 notifies the nodes 0 to 4, and 10 to 11connected to the network W3 of a cycle time that is continuous with thatin the network W1. Therefore, the nodes 10 to 11 which belonged to thenetwork W2 receive a cycle time that is discontinuous with the originalcycle time.

In other words, nodes that were connected to the network W1 havecontinuous cycle times even after the network integration. In the nodes10 and 11 which were connected to the network W2, however, discontinuityoccurs in the cycle time at time T1. For example, supposing that n=10and m=90 in FIG. 4B, the cycle time proceeds continuously as representedby “10-X, . . . 9, 10, 11, 12 . . . ” in the nodes that were connectedto the network W1, whereas discontinuity occurs in the middle of thecycle time as represented by “90-X, . . . 89, 90, 11, 12 . . . ” in thenodes that were connected to the network W2.

If discontinuity thus occurs in the cycle time, it becomes impossible toconduct processing on packets that were already stored in the receivingFIFO at the time of integration, while maintaining real time quality.Hereafter, this will be described in more detail.

FIG. 5A is a diagram showing a configuration of the receiving device 109that has been implemented before the present inventor achieves thepresent invention.

Packets TS&DT1 and TS&DT2 each with a time stamp added are stored in areceiving FIFO 107. It is supposed that in this state a cycle timecounter 101 receives a new cycle start packet CSP at time T2 (see FIG.5B) and starts counting new cycle times. In other words, it is supposedthat discontinuity has occurred in the cycle time at time T2. In thiscase, it becomes impossible for an output control unit 108 to conductprocessing on the packets TS&DT1 and TS&DT2 in the receiving FIFO 107while maintaining real time quality.

For example, it is now supposed that the packets TS&DT1 and TS&DT2 havetime stamps “10” and “11,” respectively, and the cycle time proceeds asrepresented by “1, 2, 3 . . . ” If at this time the cycle time isaltered on the way and becomes “1, 2, 3, 100, 101 . . . ,” the cycletimes “10” and “11” are skipped, and consequently the packets TS&DT1 andTS&DT2 are not subjected to proper processing. Furthermore, as a result,a packet TS&DT3 stored in the receiving FIFO 107 after the alteration ofthe cycle time shown in FIG. 5A is not subjected to proper processing,either, because the processing of the preceding packet is not finished.

On the other hand, such a problem does not occur in the presentembodiment.

FIG. 6 is a diagram showing an effect obtained according to the presentembodiment.

It is now supposed that the receiving device 10 shown in FIG. 1 isconnected to the network W2 of the cycle time M as shown in FIG. 6. Itis further supposed that this network W2 is connected to the network W1of the cycle time N at illustrated time P and discontinuity has occurredin the cycle time. In other words, it is supposed that the cycle time(network time) of the receiving device 10 proceeds as represented by“101, 102, 103, 104, 9005, 9006, 9007, 9008 . . . ”

On the other hand, it is supposed that the original cycle time (internaltime) in the receiving device 10 proceeds as represented by “1, 2, 3, 4,5, 6, 7, 8 . . . ”

It is supposed that a packet with a time stamp [105] is received by thereceiving device 10 at cycle time 101, and subsequently a packet with atime stamp [106] is received by the receiving device 10 at cycle time102 as shown in FIG. 6.

In the conventional art, cycle time proceeds as represented by “101,102, 103, 104, 9005, 9006, 9007, 9008 . . . ” as described above.Therefore, the packets with the time stamp [105] or [106] are notsubjected to proper processing.

On the other hand, in the present embodiment, the time stamp [105] isaltered to an original time stamp [5] (=(105−101)+1), and the time stamp[106] is altered to an original time stamp [6] (=(106−102)+2). And theoriginal cycle time proceeds as represented by “1, 2, 3, 4, 5, 6, 7, 8 .. . ” as described above. Therefore, the packets with the original timestamp [5] and [6] added are subjected to proper processing at theoriginal cycle time [5] and [6].

As described above, according to the present embodiment, the differencebetween the time stamp and the network time is calculated and thedifference is added to the internal time to generate the original timestamp. By controlling outputting of packets with the original time stampon the basis of the internal time, therefore, the processing can becontinued while maintaining real time quality even if discontinuityoccurs in the network time.

In the above-described embodiment, the calculation of the differencebetween the time stamp TS and the original time CT, i.e., thecalculation of the offset OS is conducted by the difference unit 13.However, this calculation may be conducted by the CPU connected to thesame bus as the receiving device. Hereafter, this will be described indetail.

FIG. 7 is a block diagram showing a configuration of a time stampcorrection system 20 according to an embodiment of the presentinvention.

A receiving device 21, a memory 22 and a CPU 23 are connected to thesame bus 30. An interface 25 is connected to an IEEE1394 network. Thereceiving device 21 is equivalent to that shown in FIG. 1. However, thedifference unit 13 is not included, and instead a difference calculationrequesting unit 24 is included. Upon being supplied with the time stampTS separated by the receiving filter 12 (see FIG. 1), the differencecalculation requesting unit 24 sends the time stamp TS and the cycletime CT at this time to the CPU 23. The CPU 23 calculates a differencebetween the received time stamp TS and cycle time CT, and returns thedifference data, i.e., the offset OS to the difference calculationrequesting unit 24. The difference calculation requesting unit 24 passesthe received offset OS to the adder 14 (see FIG. 1).

Here, the example in which the calculation of the offset OS is conductedby the CPU 23 has been shown. Besides, the processing of adding theoriginal time OT and the offset OS and thereby generating the originaltime stamp OTS may also be conducted by the CPU. Hereafter, this will bedescribed in detail.

FIG. 8 is a block diagram showing a configuration of a time stampcorrection system 29 according to another embodiment of the presentinvention.

As shown in FIG. 8, a receiving device 26 does not include the adder 14(see FIG. 1). Instead, the receiving device 26 includes an additionrequesting unit 27. Upon being supplied with the offset OS from thedifference unit 13 (or the difference calculation requesting unit 24),the addition requesting unit 27 sends the offset OS and the originaltime OT (see FIG. 1) at this time to the CPU 23. The CPU 23 adds thereceived offset OS and the original time OT, thereby generates theoriginal time stamp OTS, and returns the generated original time stampOTS to the addition requesting unit 27. The addition requesting unit 27passes the received original time stamp OTS to the data generating unit16.

By making the CPU 23 execute the calculation of the offset OS and/or thegeneration of the original time stamp OTS as hereafter described, thecircuit scale of the receiving device can be made small.

FIG. 9 is a block diagram showing a configuration of a time stampcorrection system 28 according to further another embodiment of thepresent invention.

In the embodiments described above, the original time counter 15 (seeFIG. 1) is arranged within the receiving device. In the presentembodiment, however, the original time counter 15 is arranged outsidethe receiving device.

A receiving device (first semiconductor substrate) 29, the original timecounter (second semiconductor substrate) 15, the memory 22 and the CPU23 are connected to the same bus 30. The receiving device 29 isequivalent to that shown in FIG. 1. However, the receiving device 29does not include the original time counter 15. The adder 14 and theoutput control unit 18 (see FIG. 1) in the receiving device 29 conductsprocessing by successively acquiring the original time OT from theoriginal time counter 15 connected to a bus 30.

By arranging the original time counter 15 outside the receiving deviceas described above, the circuit scale of the receiving device can bemade small. In addition, it also becomes easy to share the original timecounter 15 with other devices.

1. A time stamp correction apparatus which receives a packet includingreal time data and a time stamp, which is based on network time and tobe added to the real time data from a network, and reproduces the realtime data in the received packet on the basis of the time stamp, thetime stamp correction apparatus comprising: a packet receiving unitwhich receives a first packet including the real time data and a firsttime stamp based on the network time from the network; a network timecounter which successively generates the network time in accordance withnetwork time synchronous information input from the network; adifference calculating unit supplied with the first time stamp includedin the received first packet to calculate a difference between timeindicated by the first time stamp and the network time; a local timecounter which successively generates local time; a time stamp generatingunit which generates a second time stamp on the basis of the differenceand the local time; a packet generating unit which generates a secondpacket by adding the second time stamp to the real time data; a bufferunit which stores the generated second packet; and an output controlunit which conducts output control on the second packet in the bufferunit on the basis of the local time.
 2. The time stamp correctionapparatus according to claim 1, further comprising: a filter whichextracts the real time data and the first time stamp from the firstpacket received by the packet receiving unit, wherein the filter outputsthe extracted real time data to the packet generating unit, and outputsthe extracted first time stamp to the difference calculating unit. 3.The time stamp correction apparatus according to claim 1, wherein thedifference calculating unit calculates the difference by subtracting thenetwork time from the time indicated by the first time stamp.
 4. Thetime stamp correction apparatus according to claim 3, wherein the timestamp generating unit generates the second time stamp by adding thedifference to the local time.
 5. The time stamp correction apparatusaccording to claim 1, wherein the time stamp generating unit generatesthe second time stamp by adding the difference to the local time.
 6. Thetime stamp correction apparatus according to claim 1, wherein in thecase where the time indicated by the second time stamp in the secondpacket coincides with the local time, the output control unit outputsthe second packet.
 7. The time stamp correction apparatus according toclaim 1, wherein the network is an IEEE 1394 network.
 8. The time stampcorrection apparatus according to claim 7, wherein the packet receivingunit receives the first packet under an isochronous transfer system. 9.The time stamp correction apparatus according to claim 8, wherein thenetwork time synchronous information is a cycle start packet.
 10. Thetime stamp correction apparatus according to claim 1, wherein the realtime data is any one of image data and sound data.
 11. The time stampcorrection apparatus according to claim 1, wherein the packet receivingunit, the network time counter, the difference calculating unit, thetime stamp generating unit, the packet generating unit, the buffer unit,and the output control unit are formed on a first semiconductorsubstrate, the local time counter is formed on a second semiconductorsubstrate different from the first semiconductor substrate, and thefirst and second semiconductor substrate are connected to an identicalbus.
 12. A time stamp correction system for being used in an apparatuswhich receives a packet including real time data and a time stamp, whichis based on network time and to be added to the real time data from anetwork, and reproduces the real time data in the received packet on thebasis of the time stamp, the time stamp correction system comprising: atime stamp correction apparatus comprising a packet receiving unit whichreceives a first packet including the real time data and a first timestamp based on the network time from the network, a network time counterwhich successively generates the network time in accordance with networktime synchronous information input from the network, a local timecounter which successively generates local time, and a differencerequesting unit which requests calculation of a difference between thefirst time stamp in the first packet and the network time; and a CPUconnected to an identical bus with the time correction apparatus toexecute the difference calculation requested by the time stampcorrection apparatus and output an execution result, wherein the timestamp correction apparatus further comprises: a time stamp generatingunit which receives the execution result from the CPU and generates asecond time stamp on the basis of the received execution result and thelocal time; a packet generating unit which generates a second packet byadding the second time stamp to the real time data; a buffer unit whichstores the generated second packet; and an output control unit whichconducts output control on the second packet in the buffer unit on thebasis of the local time.
 13. The time stamp correction system accordingto claim 12, further comprising: a filter which extracts the real timedata and the first time stamp from the first packet received by thepacket receiving unit, wherein the filter outputs the extracted realtime data to the packet generating unit, and outputs the extracted firsttime stamp to the difference requesting unit.
 14. The time stampcorrection system according to claim 12, wherein the CPU calculates thedifference by subtracting the network time from the time indicated bythe first time stamp.
 15. The time stamp correction system according toclaim 14, wherein the time stamp generating unit generates the secondtime stamp by adding the difference to the local time.
 16. A time stampcorrection system for being used in an apparatus which receives a packetincluding real time data and a time stamp, which is based on networktime and to be added to the real time data from a network, andreproduces the real time data in the received packet on the basis of thetime stamp, the time stamp correction system comprising: a time stampcorrection apparatus comprising a packet receiving unit which receives afirst packet including the real time data and a first time stamp basedon the network time from the network, a network time counter whichsuccessively generates the network time in accordance with network timesynchronous information input from the network, a local time counterwhich successively generates local time, a difference calculating unitsupplied with the first time stamp included in the first packet tocalculate a difference between the first time stamp and the networktime, and a time stamp generation requesting unit which requestsgeneration of a second time stamp based on the difference and the localtime; and a CPU connected to an identical bus with the time correctionapparatus to generate the second time stamp with accordance with therequest from the time stamp correction apparatus, and output thegenerated time stamp, wherein the time stamp correction apparatusfurther comprises: a packet generating unit which receives the secondtime stamp and generates a second packet by adding the real time data tothe second time stamp; a buffer unit which stores the generated secondpacket; and an output control unit which conducts output control on thesecond packet in the buffer unit on the basis of the local time.
 17. Thetime stamp correction system according to claim 16, further comprising:a filter which extracts the real time data and the first time stamp fromthe first packet received by the packet receiving unit, wherein thefilter outputs the extracted real time data to the packet generatingunit, and outputs the extracted first time stamp to the differencecalculating unit.
 18. The time stamp correction system according toclaim 16, wherein the difference calculating unit calculates thedifference by subtracting the network time from the time indicated bythe first time stamp.
 19. The time stamp correction system according toclaim 18, wherein the CPU generates the second time stamp by adding thedifference to the local time.